The present invention relates to a simple apparatus for measuring current, such as power line current, utilizing current waveform sensing circuitry and processing circuitry including a microprocessor for computing current from current waveform measurements.
Heretofore various circuits for measuring current have been proposed and examples of some of the prior circuit proposals are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,467,858 Burnett 4,054,834 Boirat et al 4,117,399 Ono et al 4,126,825 Houston et al 4,182,983 Heinrich et al 4,206,405 Pentecost 4,209,743 Muller et al 4,291,377 Schneider et al ______________________________________
Also see the Wegner East German Pat. No. 139,914, and the Hahn West German Published Patent Application No. 23 33 907.
The Burnett U.S. Pat. No. 3,467,858 discloses circuitry including a current transformer coupled to a power line for developing a current related signal, circuitry for converting the signal to a relative high frequency pulse train much higher than the line frequency, transmitting the pulse train optically to processing circuitry where the pulse train is converted back to an electrical signal and processed utilizing rectifiers, pulse generators, ampifiers, wave shapers, counters, timers and comparators.
The Boirat U.S. Pat. No. 4,054,834 discloses an alternating current measuring device comprising an isolation transformer and bridge circuit with two light emitting diodes for developing analog light signals which are transmitted by optic fibers to two light receptors which convert half wave pulses to electrical signals which are applied to a differential amplifier. Here the instantaneous magnitude of the measured current is converted to a proportional instantaneous magnitude of light. At the receptors, the luminous intensity is measured and determines the instantaneous value of the current.
It will be appreciated that the accuracy of the Boirat device is affected by optical attenuation in the optical fibers by mismatching of the fiber terminations and splices and by electrical/optical and optical/electrical conversion gains.
The Ono U.S. Pat. No. 4,117,399 discloses a light converter including an optical effect element for use in an apparatus for measuring alternating current which counts peaks.
The Houston et al. U.S. Pat. No. 4,126,825 discloses an electronic current transducer utilizing fiber optics isolation and light signal transmission and includes a fast analog channel and a slow digital channel and a comparator which compares the slower digital signal with a light signal converted from the fast analog signal. The line current information is transmitted as a pulse train developed by a voltage to frequency converter.
The Heinrich et al. U.S. Pat. No. 4,182,983 discloses an A.C. electric energy measuring circuit which converts a di/dt analog input signal into a pulse width modulated signal responsive to the amplitude of a current component of an alternating current energy quantity to be measured.
U.S. Pat. No. 4,206,405 is directed to a watt-hour meter that permits time modulation of the power utilized whereby varying rates can be imposed upon the power user.
U.S. Pat. No. 4,209,743 discloses a circuit arrangement for measuring current at high potential utilizing a capacitor which is charged in accordance with the current to be measured. The capacitor is connected across a light emitting diode so that light pulses are generated having a frequency proportional to the current. A Zener diode and a light emitting diode (LED) are utilized in this circuit arrangement.
A similar circuit arrangement utilizing a Zener diode and LED in a voltage measuring converter is also disclosed in East German Pat. No. 139,914.
U.S. Pat. No. 4,291,377 discloses an apparatus for measuring AC power which subtracts an average value of current and voltage from sampled instantaneous current and voltage values respectively. The resultant current and voltage values are multiplied so as to obtain a power measurement substantially free of any sampling errors and any DC levels.
The West German Offenlegungsschrift Pat. No. 23 33 907 discloses a DC current measurement circuit arrangement that converts the current to a proportional pulse width or frequency modulated pulse train. The pulse train is converted to optical pulses which are transmitted optically and then converted back to electrical pulses and integrated.
As will be described in greater detail hereinafter, the current measuring apparatus of the present invention differs from all the various circuit arrangements disclosed in the prior art patents referred to above by providing for the development or generation of measurement pulses in the sensing circuitry of the apparatus which pulses are exactly at the line frequency or twice the line frequency.
In the apparatus of the present invention, it is the duration of each individual measurement pulse which is used to determine the current magnitude and such measurement pulse width measurements are highly accurate due to the high accuracy of a crystal controlled oscillator of a microprocessor control circuit forming the measurement pulse processing circuitry of the current measuring apparatus.
Also in one preferred embodiment, a low threshold Zener diode is used for generating a measurement pulse having a long duration at the low threshold during one half cycle of the current waveform and a high threshold Zener diode is used for generating a measurement pulse having a short duration at the high threshold during the other half cycle of the current waveform.